Aluminum-magnesium, or 5xxx series alloys, have a combination of good strength, weldability, and corrosion resistance that makes them ideal for ship construction and use in marine environments. However, these alloys can become susceptible to intergranular corrosion over their service life. For instance, aluminum alloys with greater than about 3% Mg (including, for example, welded 5456 and 5083 alloys) can be sensitized and become susceptible to intergranular corrosion, and thus cracking, when exposed to elevated temperatures. Thus, such alloys are not considered suitable for service above approximately 65° C.
Aluminum superstructures of, for example, surface ships, have experienced cracking due to the effects of corrosion. The structural 5456 plate, often used for such applications, has become sensitized from long-term exposure to slightly elevated temperatures and perhaps from heat associated with GMA (Gas Metal Arc) welding and, thus, more prone to intergranular corrosion. A sensitized microstructure is one in which the alloying element Mg segregates and forms beta phase precipitates (Al3Mg2) in a continuous or semi-continuous fashion along the grain boundaries. Thus, sensitization in this case is defined as a microstructure wherein Mg-rich phase precipitates along grain boundaries as Al3Mg2 (also known as beta phase), in a semi-continuous or continuous fashion or morphology. The beta phase is generally anodic to the grain interiors and brittle and, thus, plates with a sensitized microstructure are susceptible to intergranular corrosion, exfoliation, and stress-corrosion and intergranular cracking when exposed to stress and corrosive media. Once this microstructure is present, corrosion, such as, for example, from salt water, can cause intergranular corrosion and cracking.
Sensitization is not easily reversible, however it may be possible to re-dissolve the beta phase into the alloy matrix via an anneal heat treatment. However, since the 5456 plate derives its strength from work hardening, the annealed plate is softer than, for example, the H116 or H321 marine grade plate. The comparison is a tensile yield strength of approximately 37 ksi for the marine grade plates and 23 ksi for a fully annealed plate. Annealing can, of course, be done partially, and the plate strength can be made to be similar to that of the GMA weld yield strength of approximately 26 ksi.
The problem of intergranular corrosion and cracking gained new attention in 2002 after more than 200 commercial vessels built with 5083-H321 plate were found to be susceptible to intergranular corrosion [see Reference 1]. Many of these vessels required new hulls and superstructures, which led to the adoption of a new ASTM standard B928 [see Reference 2]. This standard required additional certification of aluminum alloy plates for marine use, including the use of the Nitric Acid Mass Loss Test (“NAMLT”) [see Reference 3] to better demonstrate corrosion resistance. Nitric acid dissolves the beta phase, thus causing grains surrounded by a relatively continuous network of beta phase to fall out, resulting in significant mass loss from the test sample. Unfortunately, Al—Mg alloy plate samples can pass the B928 requirements and yet, over time, the plate still develops a sensitized microstructure in service, especially in the heat affected zone of a weld [see Reference 4]. That is, at temperatures within the suitable service envelope for these alloys (e.g., below approximately 65° C.) beta phase can still precipitate on grain boundaries over long time periods.
To combat this problem, aluminum companies have tried to apply a stabilization heat treatment to aluminum plates, such that the Mg does precipitate continuously along grain boundaries. For example, during fabrication of 5xxx aluminum plate, rolling is often followed by a stabilization heat treatment. While stabilization often refers to a process developed in order to prevent age-softening, there is another stabilization treatment by which magnesium is precipitated in grain interiors or discontinuously on grain boundaries to reduce the likelihood of future sensitization [see Reference 5]. However, this practice is difficult to apply, and is not always performed correctly (or performed at all), which is evident by the number of problems realized from aluminum ship superstructures. A difficulty further arises in that the proper heat-treatment temperature range is narrow and varies with rolling practice [see Reference 6]. If the plate is treated at a temperature that is too low, beta phase will precipitate on grain boundaries and sensitization will be accelerated. If the stabilization temperature is too high, the Mg will go back into solution in the aluminum matrix, but the strain hardening that Al—Mg alloys rely on for strength will be annealed out and a significant loss in strength will result. In addition, Mg in solution is not stable and may re-precipitate to the grain boundaries over time under the right conditions.
The present inventive disclosure is directed toward overcoming one or more of the above-identified problems.